The maintenance and replacement of damaged concrete sewer pipes and related structures carries significant infrastructure liabilities for metropolitan areas all over the globe. In a 1989 survey of 89 wastewater utilities in the United States, 63% reported active corrosion prevention programs to rehabilitate concrete pipe corrosion, and 70% reported corrosion at various sites within their treatment plants. Since this time, reports of corrosion have continued to increase. Wastewater collection systems in the United States consist of about 800,000 miles of sewers and 12 million wastewater manholes. About 25% of this infrastructure is over 40 years old, and over 80% is concrete. Sewer pipes typically run below streets or developed property, so repair and replacement procedures can disrupt community economic activity. In addition, hydrogen sulfide released from wastewater collection systems can threaten environmental health as well as cause aesthetic problems.
Concrete corrosion often occurs as a result of microbially mediated sulfur cycling within wastewater collection systems. Sulfate present in wastewater is oxidized to sulfide in anoxic biofilms below the waterline. This sulfide can partition into the pipe headspace as H2S (hydrogen sulfide) gas, which serves as a substrate for acidogenic sulfur oxidizing bacteria above the waterline. These bacteria produce sulfuric acid which chemically alters the cement binder and weakens the concrete pipe. This is biogenic sulfuric acid when produced by microbes; it is as potent as any manufactured acid, and reacts with calcium and aluminum oxides in the cured cement to produce corrosion products of gypsum, ettringite, and monosulfoaluminate, which increase the concrete volume, decrease its density and undermine structural stability. Corrosion products can be washed away at higher flows, resulting in decreased mass and reduced pipe thickness.
Depending on the degree of structural deterioration, a corroded pipe must either be repaired, rehabilitated, or replaced; otherwise extreme corrosion progresses into structural failure scenarios. Concrete additives and coatings have been developed in the prior art in an attempt to impair the growth of biofilms in new concrete structures and to rehabilitate damaged structures. These prior art methods fall into three general categories: a) use of corrosion-resistant pipe materials, b) surface coatings and linings, and c) antimicrobial concrete additives.
As an alternative to concrete pipe, polymer-based pipe materials such as polyvinyl chloride (PVC), fiberglass, and high-density polyethylene (HDPE) are not susceptible to acid attack and are thus resistant to corrosion. However, the cost of these materials is significantly higher than the cost of reinforced concrete pipe, and their structural strength is inferior.
Prior to the present disclosure protective surface coatings were applied to concrete such as, for example, the inner concrete surface of concrete pipe, to prevent corrosion by placing a physical barrier between biogenic sulfuric acid and the concrete surface. Application methods include spraying, painting, and slip-lining. Adequate surface preparation and effective application are important with this rehabilitation approach, as any leaks in the coating allows corrosion to continue beneath the coating. Accomplishing such uniform coating is especially difficult in the sewer environment as a result of variable temperature, moisture, and the need to divert flow while work is conducted. Surfaces can be prepared for coating by high-pressure washing, which removes corrosion product, biofilm growth, grease and dirt. Materials used for pipe surface coatings include epoxy, polyurethane, polyurea, and coal tar. Protective coatings are intended to be resistant to acid attack, and experience less corrosion under simulated biological and abiotic corrosive conditions than uncoated concrete. Epoxy is cured with heat or through polymerization with amines or polyamides. It is durable and resistant to acid, abrasion, and moisture; resistance is correlated with high solids content. Spray-on linings of either cementitious (i.e., shotcrete) or polymeric nature are another common rehabilitation method, as they are easier to apply and cure. A popular rehabilitation method is slip-lining degraded pipe with a cure-in-place pipe (CIPP) plastic liner, which is both structurally supportive and resistant to corrosion. This method is documented under ASTM Standard F-1216. After the corrosion products are removed, a lining is placed in the sewer pipe and cured by UV light. CIPP lining materials include reinforced polyethylene, polyvinyl chloride, and carbon/fiberglass.
Antimicrobial treatments for concrete include the use of metals or other “antimicrobials” to inhibit the growth of sulfide-oxidizing microbes responsible for biogenic sulfuric acid production. For example, antibiotic-loaded fibers (MicrobanB) inhibit bacterial growth using triclosan—an antibiotic that is also used in personal care products. As another example, zeolites are aluminosilicate adsorbent materials that are used in the corrosion industry to (slowly) deliver toxic heavy metal ions such as copper and silver to the concrete surface over a pipe's service life. Concrete mortars treated with either technology and inoculated with sulfide-oxidizing cultures exhibit reduced levels of adenosine triphosphate (ATP)—an indicator of microbial activity—when compared to untreated cement mortar. Silver zeolites also inhibit the growth of Acidithiobacillus thiooxidans and other bacterial cultures. Heavy metal oxides (copper and silver) mixed with commercial epoxy have been shown to inhibit sulfate-reducing cultures, but have not been tested with sulfide-oxidizing cultures. These methods fare well in laboratory and small-scale tests, but are typically more expensive than epoxy or polyurea lining Another product called ConMicShield uses silicone quaternary ammonium salts to kill bacteria electrostatically and has been widely applied to inhibit corrosion-causing bacteria. ConMicShield can be added to concrete mix prior to curing or applied retroactively using shotcrete.
The foregoing examples of the related art and limitations related therewith are intended to be illustrative and not exclusive. Other limitations of the related art will become apparent to those of skill in the art upon a reading of the specification and a study of the drawings.